The present invention relates to a radiological scanning apparatus, particularly for radio diagnosis, for forming an image of an object or zone of a patient to be examined on a film, while scanning this object with a flat fan shaped X ray beam.
The apparatus for forming a radiologial image of an object generally include an X ray source, sending a substantially uniform flow of X rays over the whole surface of the object whose image it is desired to form. The X rays pass through the object while undergoing variable attenuation, and produce an image on a receiver sensitive to the X rays.
The receiver may be a film in which the effect of the X rays produces a modification of the material forming its sensitive surface, such as the creation of crystal nuclei, creation of traps, or more generally any transformation giving rise to a local modification of a physical property which may be measured by an appropriate analysis means; such film consisting for example of a photographic plate the darkening of which depends on the X ray dose received at each point, or else a plate of a photostimulable luminescent material, the luminescence of each point being proportional to the X ray dose received.
Another type of receiver may be formed by a two dimensional network of cells sensitive to the X rays, of small size, each cell delivering an electric signal proportional to the X ray dose received.
In conventional radio diagnosis apparatus, the X ray dose received at a point of the receiver corresponds, on the one hand, to the amount of X rays which have not been absorbed by the object over a path in a straight line from the X ray source to the receiver, and, on the other hand to the amount of X rays diffused by all the points of the objects subjected to the X ray radiation. Thus, the receiver records two images:
the first image is formed by the direct rays propagating in a straight line, this image having a resolution which only depends on the dimension of the X ray source;
the second image is formed by the diffused X rays; this image is very fuzzy and considerably reduces the quality of the overall image.
Radiological scanning apparata reduce the influence of the diffused X rays. In these apparata, the X ray source illuminates the object through a collimator or slit which gives to the x rays the form of a flat so called "fan shaped" beam. The beam then passes through a second slit slightly narrower than the projection of the first slit or collimator on the plane of the second slit.
The flat fan shaped beam then exposes a receiver which may be a flat receiver as in the scanless system, or an elongate receiver limited to the projection of the second slit on the plane of this receiver.
In this configuration, the object is disposed between the first and second slits only a fraction of the object is exposed to the X rays. This fraction is in the form of a slice defined by the intersection of the fan shaped beam and the object; thus, a point of the receiver exposed to the direct X rays is only exposed to the diffused X rays by the exposed fraction of the object, which forms a reduction of the diffused X rays all the greater the narrower the first slit, that is to say that the beam is flat.
To expose any object, the fan shaped beam is moved with respect to the object so as to successively expose all the fractions thereof in a scanning movement, the complete image of the object being obtained at the end of such scanning.
The systems which use the scanning technique offer a considerable improvement in the image quality. But up to now, one of the limits in use of this technique is due to the receivers, particularly when it is desired to form simultaneously several images of the same zone of the object to be examined. It may in fact be interesting to obtain two simultaneous images of the object, obtained by two flat fan shaped beams whose sources have for example a different position, so as to obtain different images which provide stereoscopic information; another case may for example be that where the flat fan shaped beams have different energy spectra, the different images of the same object at different energies may then provide additional information about the nature of the tissues or materials forming the object.
Since the fan shaped beams have a relatively small thickness, it is possible in a scanning system to dispose several pairs of slits, so as to form several fan shaped beams which fan the object one after the other. At the present time, the receivers of the film type mentioned above do not lend themselves to such a configuration, particularly because of their size which is too great, considered in the scanning direction and up to now two successive fan shaped beams superimposed their image on a single film.
The problem of the size of the receiver may be settled by using receivers of the type having detection cells, such as already mentioned. The detection cells are then organized so as to form a detection strip whose detection plane, exposed to the beam, has an elongate form in the direction of the plane of the fan shaped beam. The detection strip is moved during scanning, so as to merge permanently with the projection of the second slit mentioned above as being interposed between the object and the receiver.
The disadvantage of this solution resides in the fact that it does not allow an image to be obtained having a definition as good as the image obtained by a film, this definition being related to the size of its cell and to the number thereof.
If the second slit is of a size smaller than the resolution of the desired image, the detection strip may be formed by a single row of detection cells. When the second slit is greater than the desired resolution, the detection strip must be formed of several rows of cells, which involves using a device for transferring the information from one row of cells to the adjacent row of cells, at the moving speed of the detection strip.
This arrangement, while not allowing the quality of the image obtained by a film to be reached, adds to the complexity of the means required for reforming an image from the information delivered by each cell during scanning. It should be noted that these means form large and particularly costly equipment with respect to the means required for using a film.